THE BEAN FAMILY. 275 



work on the flowers. The following notes on the chief types 

 of flower-mechanism may be useful : 



(a) Stamens and stigma emerge from keel when it is de- 

 pressed, and return inside it again when the bee flies away ; 

 the same flower may therefore be visited several times : 

 Clover, Sainfoin, Laburnum, etc. 



(6) Keel-petals joined above as well as below, leaving a 

 small opening at the tip ; pollen shed into upper end of keel 

 before flower opens; five of the stamens have filaments 

 thickened below the anthers, and the piston thus formed 

 pushes out some of the pollen each time the keel is depressed : 

 Bird's foot Trefoil (Lotus). The other five stamens are 

 shorter and shrivel up after they have shed the pollen, which 

 is carried by the long stamens (those with the thickened 

 ends) into the tip of the keel. A somewhat similar mechan- 

 ism is found in the Lupin and in Lady's-fingers (Anthyllis). 



(c) Upper edges of keel-petals at first joined, and pollen 

 forced out at tip, as in b, but later on the keel splits open, 

 and stamens and stigma emerge as in a : Rest-harrow (no 

 honey in flower, therefore stamens all united). 



(d) Like Broad Bean, with tuft of hairs on the style 

 which sweeps the pollen out of the keel when the latter is 

 depressed : most Vetches and Peas. 



(e) Mechanism as in d, but stamens long and coiled up 

 within a coiled keel : Kidney Bean, Scarlet Eunner. 



(/ ) Stamens and style tightly held, more or less coiled up, in 

 the keel under tension, and springing out violently when keel 

 is depressed: Grorse, Broom, Petty Whin (Genista Anglica), 

 Dyer's- weed (G. tinctoria), Lucerne and other Medicks 

 (Medicago) . 



Kecent observations on the flowers of Papilionaceae have led 

 to the generalisation that while many perennial kinds have self- 

 sterile flowers (see Art. 263) , all the self -fertile Idnds are annuals. 

 Some of the latter, in fact, are self -pollinated before the flower- 

 bud opens (e.g. Sweet Pea, Garden Pea). The annuals cannot 

 afford to be self -sterile, because they depend on each year's 

 crop of seeds for their very existence, whereas perennial plants 

 can safely run the risk of not being pollinated at all on the 

 chance of securing cross-pollination now and then. 



